This invention relates to thermoplastic polyolefins. More particularly, this invention relates to copolymers of ethylene with C4+ olefin monomers which typically contain blocky structures as shown by peak melting temperatures above those measured in corresponding random copolymers with similar monomer composition, and to methods of production by use of fluxional metallocane catalysts.
Thermoplastic olefin polymers represent a significant worldwide market with millions of tons of these polymers produced and sold each year. Copolymers of ethylene with C4+ monomers are a substantial fraction of the worldwide olefin polymer production, especially in use as films. Although the bulk of ethylene polymers are thermoplastics, there is a growing further need for elastomeric thermoplastic olefin polymers.
Copolymers of ethylene with higher (C4+) olefin monomers are well known and used in the art. Among these are linear low density polyethylenes conventionally produced as a copolymer of ethylene with 1-butene or 1-octene using traditional Ziegler-Natta catalyst systems. These materials typically have broad polydispersities, and broad composition distributions.
Some of these ethylene-C4+ copolymers have a particularly broad range of application as elastomers. There are generally three families of elastomers made from such copolymers. One class is typified by ethylene-propylene copolymers (EPR) which are saturated compounds, optimally of low crystallinity, requiring vulcanization with free-radical generators to achieve excellent elastic properties. Another class of elastomer is typified by ethylene-propylene terpolymers (EPDM), again optimally of low crystallinity, which contain a small amount of a non-conjugated diene such as ethylidene norbornene. The residual unsaturation provided by the diene termonomer allows for vulcanization with sulfur, which then yields excellent elastomeric properties.
Yet a third class is typified by ethylene-alpha olefin copolymers of narrow composition distribution which possess excellent elastomeric properties even in the absence of vulcanization. For example U.S. Pat. No. 5,278,272, to Dow describes a class of substantially linear polyolefin copolymer elastomers with narrow composition distribution and excellent processing characteristics. These are produced with conventional metallocene-based catalyst systems which have narrow polydispersities, narrow composition distributions and melting point ranges corresponding to random copolymers. Representatives of these metallocene copolymers are ethylene/1-butene copolymers sold as Exact"" brand by Exxon Chemical and ethylene/1-octene copolymers sold as Engage"" brand by Dow Chemical. One of the limitations of these latter class of elastomers is their low melting temperature which limits their high temperature performance.
This invention relates to copolymers of ethylene with C4+ olefin monomers which may be thermoplastics or elastomers. Particularly, these copolymers typically are formed from a fluxional catalyst system which creates properties consistent with a blocky structure. A polymer chain with a blocky structure will contain segments of differing compositional microstructure. Thus, in an ethylene/hexene copolymer of this invention, the evidence indicates ethylene homopolymer blocks are distributed in the polymer chain with adjacent segments of ethylene/hexene copolymer. Since ethylene homopolymer segments will form regions of polyethylene crystallinity while ethylene/hexene copolymer segments will be amorphous, the polymer as a whole contains regions of polyethylene crystallinity interspersed with amorphous regions to a greater extent than would be observed in a copolymer of ethylene with randomly dispersed comonomer. Typically the upper peak melting temperatures for the copolymers of this invention are higher than corresponding random copolymers, although the melting transition is relatively broad and typically has multimodal or bimodal melting temperature peaks. This data indicates that the polymers of the present invention contain larger, more thermodynamically stable crystals and longer ethylene sequences than that present in a random ethylene polymer.
The broad melting range exhibited by the copolymers of this invention extending to higher melting temperatures than random polymers of similar branching, indicates the former crystallize to give a broader range of crystal types (high and low melting). The high melting crystals are a result of the non-random comonomer incorporation allowing the formation of longer runs of ethylene homopolymer sequences than occurs in random versions. The comonomer, such as hexene, interrupts crystallization and, thus, the largest and most stable crystal into which a polymer chain can crystallize is defined by the longest ethylene unit run length present.
Further, the copolymers of the invention show a narrow compositional distribution among fractions separated by crystallinity or molecular weight. The copolymers of this invention show improved optical properties, such as clarity and reduced haze in films, as follows from a narrower composition distribution. The copolymers of the invention also exhibit a relatively broad polydisperity, a property which results in superior processibility.
In another aspect of this invention, the olefin copolymers of the invention are characterized by low glass transition temperatures, melting points above about 90xc2x0 C., high molecular weights, and a narrow composition distribution between chains. The copolymers of the invention are novel reactor blends that can be sequentially fractionated into fractions of differing crystallinities. These fractions nevertheless show compositions of comonomers which differ by less than 15% from the parent reactor blend. The invention also relates to a process for producing such copolymers by using unbridged fluxional metallocene catalysts that are capable of interconverting between states, each state having different copolymerization characteristics, i.e., each state having a different relative rate of insertion of a given ethylene or C4+ monomer into the growing copolymer chain and preferential selectivity for different monomers under particular reaction conditions.
An important object of this invention is to provide methods of production of a novel class of polyolefin copolymers with a combination of commercially important physical characteristics, including: a molecular weight distribution, Mw/Mn greater than 2, a narrow composition distribution,  less than /=15%, high melting point index, melting points greater than about 90xc2x0 C., and typically above the melting temperature of a random copolymer having the same monomer unit composition. It is a further object of this invention to produce a novel family of crystallizable, high-melting polyolefin copolymers having a narrow composition distribution where the melting point of the polymer is greater than about 90xc2x0 C. It is a further object of this invention to produce a class of high-melting, multiblock, blend, and multiblock/blend polyolefin copolymer elastomers. These novel polymers are useful as thermoplastic materials as well as compatibilizers for other polyolefin blends.
The copolymers of this invention are produced using a new family of fluxional metallocene-based catalysts first described in U.S. Pat. No. 5,594,080, incorporated by reference herein. These catalysts produce blocky structures in the polymer chain which yield polymer products having a combination of properties which is advantageous for multiple use applications including films. This combination of properties include a narrow composition distribution, broad polydispersity, and a broad melting transition with an upper melting peak which typically is higher than a randomly distributed copolymer with the same monomer unit composition.
Products made from the copolymers of this invention benefit from the products improved processibility of the polymer, higher temperature performance range, and uniformity. Applications include films, including heat sealable films, and molded products. More particularly with respect to films, films can be produced with improved optical properties such as low haze and improved clarity.
The copolymers of the Invention can be characterized as copolymers of ethylene and at least one comonomer containing at least 4 carbon atoms having a polydispersity greater than 2, a broad melting point transition as measured by differential scanning calorimetry, and a narrow composition distribution. Ethylene/C4+ copolymers of the invention also may show at least one peak melting point above the peak melting point of a random copolymer of the same monomer unit composition. These copolymers are made by contacting ethylene and a comonomer under polymerization conditions in the presence of a suitable fluxional metallocene catalyst system.